1. Field of the Invention
This invention relates to a touch entry system for detecting the presence of an element in an irradiated field and, more particularly, to a touch entry system and circuit for use in a touch entry system for detecting the intrusion and relative location of an element present within an irradiated field generally adjacent to an electronic display, in which the irradiated field is formed by a plurality of light emitters and light detectors.
2. Description of the Prior Art
Along with the continued and increasing use of video displays has been the problem of the operator/machine interface. Traditionally, control of displayed information or cursors has been via the use of a keyboard. More recently, however, a number of devices allow an operator to directly interact with a video display. These devices have included light pens, desk-type mouse controllers, or touch input devices, such as switch matrixes or opto-electronic matrices. While switch-type overlays, placed adjacent to a video display, are generally inexpensive to apply and to utilize, they are also susceptible to contact wear as well as the distortion of the video information which is presented to the viewer or operator, particularly in situations involving a high degree of use. Opto-electronic matrix schemes utilizing light which is generally in the infrared region, however, create a matrix which is invisible to the viewer or operator and, therefore, do not distort the video information displayed and is not subject to wear in high use environments. Systems utilizing opto-matrix frames are well known in the art. An example of such a system includes U.S. Pat. No. 4,267,443 entitled Photoelectric Input Apparatus. U.S. Pat. No. 4,761,637 discloses a touch input device having individually addressable emitters and detectors in separate arrays.
Normally touch input devices employ a voltage source of five volts in order to support the logic normally used to perform the various functions in a touch input device. It is one of the objects of this invention to improve the activation of a plurality of light emitters using only a single five volt source. Whereas conventional touch input devices have individually addressable emitters such as LEDs, in which a five volt source provides the energy used to drive not only each emitter but also series elements consisting of a current source and a current sink used in addressing each individual emitter. Therefore it is the object of this invention to provide a current source having sufficient energy to drive the high resistance LED and addressing circuitry using only a single five volt voltage source for the touch input device. It is also an object of this invention to provide a suitable driving current for the LED without increasing the electrical noise in the touch input system. If a conventional charge pump type device or transformer means were employed in a touch input system, this would generate electrical noise which would complicate the detection of light beams.
With conventional infrared touch input systems, the use of a single five volt source with a light emitting diode and a source driver and a sink drive, it is not always possible to generate sufficient light from each of the multiple light emitting diodes used in a touch input device. The voltage drops across the sink and source drivers and across selected LEDs will in some cases be greater than average thus leading to a low signal output from that LED. This factor is largely due to the variations in the saturation voltages of the source and sink devices and the electrical performance of light emitting diodes manufactured using current technology. This factor greatly complicates the manufacture of a touch input system, particularly when a large number of LEDs are employed in the touch input system. At present, a touch input system using a single five volt energy source must be constructed and tested to determine if the light energy emitted by each light emitting diode is sufficient for detection by each associated photodetector. With current technology, many light emitting diodes driven from a five volt source possess adequate output for use in a infrared touch input system. It is therefore an object of this invent to provide a touch input device in which additional current can be supplied to each light emitting device using a single five volt power source, so that each light emitting diode or other light emitting element will supply sufficient light for detection by light detectors spaced at the opposite end of an irradiated display surface.
It is another object of this invention to provide a generally stable current so that the current though each LED is normalized to reduce variations in current through the various LEDs in the individually addressable matrix. Therefore the light levels around the LED matrix tend to be more constant.
Another object of this invention is to provide a touch input system in which each LED does not have to be driven by a current which would otherwise be necessary to drive the lowest light output LED of the matrix, thus increasing the light emitted by the LEDs which have inherently better performance. Such excessive light can cause problems in the operation of a touch input system. Using excessive current to drive LEDs also results in degradation of individual LEDs over time.